Like many of the most famous objects in the sky, globular
cluster Messier 10 was of little interest to its discoverer. Charles Messier,
the 18th century French astronomer, cataloged over 100 galaxies and clusters,
but was primarily interested in comets. Through the telescopes available at the
time, comets, nebulae, globular clusters and galaxies appeared just as faint,
diffuse blobs and could easily be confused for one another.

Only by carefully observing their motion — or lack of it — were astronomers able
to distinguish them: comets move slowly relative to the stars in the
background, while other more distant astronomical objects do not move at all.

Messier’s decision to catalog all the objects that he could find, and that were
not comets, was a pragmatic solution which would have a huge impact on
astronomy. His catalog of just over 100 objects includes many of the most
famous objects in the night sky. Messier 10, seen here in an image from the
NASA/ESA Hubble Space Telescope, is one of them. Messier described it in the
very first edition of his catalog, which was published in 1774 and included the
first 45 objects he identified.

Messier 10 is a ball of stars that lies about 15,000 light-years from Earth, in
the constellation of Ophiuchus (The Serpent Bearer). Approximately 80
light-years across, it should therefore appear about two thirds the size of the
moon in the night sky. However, its outer regions are extremely diffuse, and
even the comparatively bright core is too dim to see with the naked eye.

Hubble, which has no problems seeing faint objects, has observed the brightest
part of the center of the cluster in this image, a region which is about 13
light-years across.

This image is made up of observations made in visible and infrared light using
Hubble’s Advanced Camera for Surveys. The observations were carried out as part
of a major Hubble survey of globular clusters in the Milky Way.

A version of this image was entered into the Hubble’s Hidden Treasures Image
Processing Competition by contestant flashenthunder. Hidden Treasures is an
initiative to invite astronomy enthusiasts to search the Hubble archive for
stunning images that have never been seen by the general public. The
competition has now closed and the results will be published soon.